It is known in the fuel cell art to evaporatively cool fuel cells, thereby deriving the benefit of the heat of vaporization, in contrast with conveying sensible heat to circulating water passing through the cells or coolant passing through coolant plates.
Referring now to FIG. 1, an evaporatively cooled fuel cell power plant 36 of U.S. Pat. No. 7,579,098 includes a stack 37 of fuel cells 38 which are vertically disposed.
Fuel from the source 41 is provided to a fuel inlet 42 and flows to the right in a first fuel pass, as indicated by the bold arrow 43, to a fuel turn manifold 44. The fuel gas then flows downwardly and into a second fuel pass of the fuel flow fields, wherein the fuel gas flows to the left as indicated by the bold arrow 45. From a fuel outlet 47, the fuel may flow through a recycle pump 48 (perhaps with valves not shown) back to the fuel inlet 42, and may be periodically purged to ambient through a valve 49, all as is known in the art. Single pass, triple pass or other fuel flow configurations may be used.
In FIG. 1, process air is provided by a pump 52 to an air inlet 53, and the air flows upwardly through the oxidant reactant gas flow channels of the fuel cells 38, as indicated by the hollow arrow 54. From a process air outlet 57, the air flows in a conduit 58 to a condenser 59, which in a vehicle may be a conventional radiator. The drier exit air is passed through an exhaust 62. The condensate from the condenser 59 may be conducted for accumulation in a reservoir 64, which is connected by a water return conduit 65 to a water inlet 66. The water then flows through fluid conduits, typically minute passageways 67, into each of the fuel cells 38; the passageways 67 may terminate in a vent manifold 68, from which removal of gas from the passageways is provided through a vent, such as a porous hydrophobic-plug vent 69; or, when suitable in any given case, the passageways may be dead-ended, or they may be connected to a micro pump at the vent 69, as known.
Although there is a water inlet 66, there is basically no water outlet, the water is simply present in each fuel cell as described more fully with respect to FIG. 2. In FIG. 2, fuel cells 38 each comprises a conventional membrane electrode assembly 72, which includes an electrolyte with anode and cathode catalysts on opposite sides thereof and may or may not include a gas diffusion layer on one or both electrodes.
In FIG. 2, fuel reactant gas flows through channels 74 in fuel reactant gas flow field plates 75, which includes grooves 76, that together with grooves 77 of an adjacent fuel cell, form minute water passageways 78. On the cathode side, an oxidant reactant gas flow field plate 81 includes process air flow channels 82 and grooves 83 which, with grooves 84 on an adjacent fuel cell, together form minute water passageways 85.
To prevent flooding, the reactant gases are typically a few Kilopascals (about one-half psi) higher than the pressure of water in the passageways. This will naturally occur as a consequence of the air pump 52 generally causing the air to be that much above atmospheric pressure, and the pressure of the fuel is easily regulated, as is known. In FIG. 1, the water in the conduit 65 may be at atmospheric pressure but could be at a pressure other than atmospheric, provided the reactant gases have a slightly higher pressure.
The water passageways may be formed other than by matching grooves as shown. Water passageways 67 may be provided in only one of the reactant gas flow field plates 75, 81.
The reactant gas flow field plates 75, 81 appear to be the same as water transport plates, sometimes referred to as fine pore plates, in a fuel cell power plant which utilizes significant water flow through the water transport plates, with external water processing, as is disclosed in U.S. Pat. No. 5,700,595. However, because there is about a one hundred-to-one improvement in cooling effectiveness per volume of water when evaporative cooling is used, in comparison with the sensible heat water flow cooling of the aforesaid '595 patent, the water flow channels in the prior art have cross sections which are several tens of times larger than the cross sections of the water passageways 78, 85 in FIG. 1. In addition, the spacing of the lateral portions of the water passageways 78, 85 (shown at each juncture of the fuel cells in FIG. 3) may be separated by a distance which is several times greater than the spacing between lateral portions of water flow channels in sensible heat, water flow cooling systems, as in the aforesaid '595 patent. The small cross section of the water passageways 78, 85, and the large distance between successive lateral portions thereof permit the thickness of the reactant gas flow field plates 75, 81 to be reduced by about one-third.
The evaporatively cooled fuel cell of the art as described with respect to FIGS. 1 and 2 has since enjoyed additional improvements and variations. Such a fuel cell combination is very advantageous, as described hereinbefore. However, the manufacture of the plates, particularly if they have grooves on both sides, is expensive both in terms of the materials used and the machining required to achieve suitable plates that are within tolerance. Because the fuel cells are separated one from the other, thereby avoiding crossover of fuel into the oxidant plates and/or oxidant into the fuel plates, bubble pressure in the fuel and oxidant plates 81, 75 must be carefully controlled such that water will flow through the plates, but gas will not. The porosity, pore size and pore volume is therefore critical as well. To ensure that flow of water into the reactant gases is avoided, the coolant water typically is one or several psi's (0.7 or 1.5 kPa's) below the reactant gas pressure.
In order to ease manufacturing processes, reduce cost of wasted material, and achieve adequate separation of the fuel cells and humidification of the anode side and the proton exchange membrane, the use of end milling is typical.
Flow field plates, or a separator plate, which are more easily manufactured and do not require bubble pressure tolerances would be advantageous.